Method of case hardening

ABSTRACT

A method of case hardening an article formed of titanium, zirconium or an alloy of titanium and/or zirconium is disclosed. First, the article is heat-treated in an oxidizing atmosphere at a temperature in the range of 700 to 1000° C. so as to form an oxide layer on the article. Then, the article is further heat-treated in a vacuum or in a neutral or inert atmosphere at a temperature in the range of 700 to 1000° C. so as to cause oxygen from the oxide layer to diffuse into the article.

This invention relates to a method of case hardening and is moreparticularly concerned with a method of case hardening an article formedof titanium, zirconium or an alloy of titanium and/or zirconium.

In engineering applications, when a surface is subjected to a highcontact load by another body, internal stresses are developed below thesurface, the so-called Hertzian stresses. These stresses reach a maximumat a certain depth below the surface. Consequently, in order towithstand such stresses, it is necessary for a case-hardened layer toprovide increased strength (and therefore hardness) down to at leastthat depth. At the same time, it is desirable to avoid excessivehardness at the surface itself as this could cause embrittlement. Toreconcile these requirements, it is generally preferred to produce ahardness profile, in the direction normal to the surface, which has asigmoid shape (see, for example, the OD curve in accompanying FIG. 2),consisting of a region of relatively high hardness maintained to acertain depth below the surface before dropping more steeply and thengradually to the hardness of the untreated core material.

Both theoretical and experimental work has shown that significantimprovements in the load-bearing capacity of a hard coating/substructure system can be achieved provided that, in addition to a highinterfacial adhesion strength, the substrate can firmly withstand theapplied load without appreciable plastic deformation. This means thatdeep case surface engineering processes are beneficial for subsequenthard thin coatings on titanium alloys in view of their inherent lowyield strengths and low elastic moduli. However, most titanium alloys,unlike ferrous materials, cannot be hardened to a great extent byconventional surface engineering techniques since there is no hardeningreaction in titanium alloys comparable to the martensite transformationin ferrous materials. Notwithstanding the fact that titanium alloys canbe deeply hardened by electron beam surface alloying, it is stilldifficult in practice to achieve controlled reproducibility ofcomposition in the alloyed surface layer. Oxidising titanium alloys at ahigh oxidation temperature for an extended period of time can alsoproduce a deep hardened case. However, simple oxidation at highertemperatures (greater than 700° C.) is prone to the formation of severescaling, resulting in a crumbly surface oxide layer. The presentinvention relates to a method which avoids this by oxidation treatmentat an elevated temperature effected for a relatively short period oftime, followed by a subsequent heat treatment operation.

A method of surface hardening titanium by oxygen is disclosed by A.Takamura (Trans JIM, 1962, Vol. 3, pages 10-14). In one of the methodsdisclosed by Takamura, samples of commercial titanium are annealed,polished and degreased and are then oxidised in dry oxygen at 850° C.for 1 or 1.5 hours. A thin oxide scale is formed on the surface of thesamples. Then, the thus-oxidised samples are subjected to a diffusiontreatment at 850° C. for 24 hours in argon so as to cause oxygen todiffuse into the sample. In other methods disclosed by Takamura, theoxidised samples are diffusion treated first in argon and then innitrogen or are diffusion treated in nitrogen. In no case, however, isthe desirable sigmoid-shaped hardness profile achieved.

It is an object of the present invention to provide a process which ismore capable of achieving the desirable sigmoid-shaped hardness profilethan the last-mentioned publication.

According to a first aspect of the present invention, there is provideda method of case hardening an article formed of titanium, zirconium oran alloy of titanium and/or zirconium, said method comprising the stepsof (a) heat-treating the article formed of titanium, zirconium or alloyof titanium and/or zirconium in an oxidising atmosphere containing bothoxygen and nitrogen at a temperature in the range of 700 to 1000° C. soas to form an oxide layer on the article; and (b) further heat-treatingthe article in a vacuum or in a neutral or an inert atmosphere at atemperature in the range of 700 to 1000° C. so as to cause oxygen fromthe oxide layer to diffuse into the article.

According to a second aspect of the present invention, there is provideda method of case hardening an article formed of titanium, zirconium oran alloy of titanium and/or zirconium, said method comprising the stepsof (a) heat-treating the article formed of titanium, zirconium or alloyof titanium and/or zirconium in an oxidising atmosphere at a temperaturein the range of 700 to 1000° C. so as to form an oxide layer on thearticle; and (b) further heat-treating the article in a vacuum or in aneutral or an inert atmosphere at a temperature in the range of 700 to1000° C. so as to cause oxygen from the oxide layer to diffuse into thearticle whereby to produce a sigmoid-shaped hardness profile.

The time for heat-treatment in step (a) is relatively short and depends,inter alia, upon the nature of the oxidising medium and the intended useof the article. Typically, the time may be, for example, from 0.1 to 1hour, preferably 0.3 to 0.6 hour.

The heat-treatment in step (a) is conveniently effected at atmosphericpressure.

Steps (a) and (b) may be repeated at least once.

In the method according to said second aspect of the present invention,the oxidising atmosphere in step (a) preferably comprises oxygen as wellas nitrogen, as this improves the adhesion of the predominantly oxidescale thus formed.

In the first and second aspects of the present invention, the oxidisingatmosphere in step (a) is preferably air. The temperature in step (a) ispreferably 700 to 900° C., more preferably 800 to 900° C., and mostpreferably about 850° C.

The temperature in step (b) is preferably 700 to 900° C., morepreferably about 800 to 900° C., and most preferably about 850° C. It ismost preferred to effect treatment step (b) in a vacuum, in which casethe pressure is preferably not more than 1.3×10⁻² Pa(1×10⁻⁴ Torr) Pa,and is conveniently about 1.3×10⁻⁴ Pa (1×10⁻⁶ Torr). The use of a vacuumis much preferred because it reduces the risk of unwanted contaminantsbeing accidently introduced into the surface of the article during step(b).

In particular, it is important to prevent gaseous oxygen from reachingthe solid surface during step (b) where it may dissolve or react so asto cause excessive hardness and potential embrittlement. Where the heattreatment in step (b) is effected in an inert or neutral atmosphere, anynon-oxidising and non-reducing atmosphere may be employed, such as argonor other inert gas, provided that it contains no or only a low partialpressure of oxygen.

The time required for the heat treatment in step (b) is typically in therange of 10 to 50 hours and may conveniently be about 20 to 30 hours.

It is within the scope of the present invention to follow the treatmentsteps (a) and (b) with any of a variety of subsequent treatments orprocesses to reduce friction. In particular, it is within the scope ofthe present invention to follow the method of the present invention withthe treatment method disclosed in our copending PCT Publication No.WO98/02595 for improving the tribological behaviour of a titanium ortitanium alloy article. Such process basically involves the gaseousoxidation of the article at a temperature in the range of 500 to 725° C.for 5.0 to 100 hours, the temperature and time being selected such as toproduce an adherent and essentially pore-free surface compound layercontaining at least 50% by weight of oxides of titanium having a rutilestructure and thickness of 0.2 to 2 μm on a solid solution-strengtheneddiffusion zone where the diffusing element is oxygen and the diffusionzone has a depth of 5 to 50 μm.

The present invention is applicable to commercially pure grades oftitanium, titanium alloys (α,α+β, or β alloys), commercially pure gradesof zirconium, zirconium alloys and to alloys of zirconium and titanium.

Where the article is required to have good fatigue properties, it may besubjected to a mechanical surface treatment, such as shot peening, afterheat treatment in order to restore the fatigue properties which may bereduced by the heat treatment operation.

According to a third aspect of the present invention, there is providedan article formed of a metal or alloy selected from titanium, zirconium,alloys of titanium and alloys of zirconium, said article having ahardened metallic case, strengthened by diffused oxygen; wherein thearticle has a sigmoid-shaped hardness profile across said hardened case.

Preferably, the depth of the hardened case is greater than 50 μm, and istypically in the range 200 to 500 μm, but may be as great as 1 mm.

A further layer of low-friction material, for example, a nitride,diamond-like-carbon or an oxide layer as described in our co-pending PCTPublication No. WO98/02595, may be provided on top of the hardened case.

In the accompanying drawings:

FIG. 1 is an SEM micrograph showing the overall microstructure of asample of an oxygen-diffused (OD) Ti6Al4V material treated in accordancewith the method of the present invention,

FIG. 2 is a graph showing microhardness profiles for the OD Ti6Al4Vmaterial produced in accordance with the present invention and for othersurface-treated articles formed of the same material (Ti6Al4V),

FIG. 3 is a graph showing the effect of OD treatment and OD plus shotpeening (OD+SP) on the fatigue properties of Ti6Al4V,

FIG. 4 is a graph showing microhardness profiles for OD C.P titaniummaterial, produced in accordance with the present invention,

FIG. 5 is a graph showing a microhardness profile for OD Timet551produced in accordance with the present invention, and

FIG. 6 is a graph showing a microhardness profile for OD Timet10-2-3material, produced in accordance with the present invention.

Samples of Ti6Al4V in the form of cylindrical coupons of 5 mm thickness,cut from a 25 mm diameter bar were used. The samples were thenthoroughly cleaned and subsequently thermally oxidised at 850° C. for 30minutes in air in a muffle furnace. After being allowed to cool, thesamples were subjected to a further heat treatment operation at 850° C.for 20 hours in a vacuum furnace (about 1.3×10⁻⁴ Pa=about 10⁻⁶ Torr).Alternatively, the steps of (a) thermal oxidation and (b) further heattreatment can be carried out in a single vacuum furnace, step (a) beingeffected in air and step (b) being effected at 1.3×10⁻⁴ Pa afterevacuation of the air.

It was noted that, after thermal oxidation at 850° C. for 30 minutes,the samples had a dark brown appearance. However, this changed to silverfollowing the further heat treatment operation. The metallography of theoxygen-diffused treated sample is shown in FIG. 1. A hardened layer wasproduced which was which was estimated from the transition in morphologyto have a depth of about 300 μm and appeared (from the different etchingeffects) to consist of two sub-layers, the first sub-layer having adepth of about 80 μm and the second sub-layer, lying under the firstsub-layer, having a depth of about 220 μm.

A typical microhardness profile for the above-treated samples isillustrated in FIG. 2 where, for comparison purposes, microhardnessprofiles are also given for samples of the same Ti6Al4V material treatedby one of three processes, namely oxidation at 850° C. for 30 minutes,oxidation at 850° C. for 20.5 hours and plasma nitriding at 850° C. for20 hours in an atmosphere of 25% N₂ and 75% H₂. It is notable that theOD material treated in accordance with the present invention showed thedesired sigmoid hardness profile with a more pronounced hardening effectin terms of higher hardness and deep-hardened zone than the thermallyoxidised material with the same thermal cycle (850° C./20.5 hours). Themicrohardness profile for the OD material in accordance with the presentinvention is in good agreement with the observed microstructuralfeatures illustrated in FIG. 1.

As can be seen from FIG. 2, the OD samples produced in accordance withthe present invention had a high hardness (greater than 700 HV_(0.05))in the first 80 μm and a total hardened layer of about 300 μm in depth.

As can be seen from FIG. 3, OD treatment in accordance with the presentinvention reduces the fatigue properties of Ti6Al4V. However, thereduction in the fatigue limit was totally restored and slightlyelevated by about 30 MPa over the untreated material by shot peening. Inthis particular case, the shot peening was effected using C glass shotwith an Almen density of 0.15-0.029N.

As noted above, the samples treated in accordance with the presentinvention possessed a significantly greater depth of hardening effectthan a direct oxidation treatment at the same temperature and for thesame total time (850° C./20.5 hours). This means that the treatment inaccordance with the present invention not only avoids the formation ofan undesirable scale, which always occurs as a result of oxidationtreatment at high temperature, but also confers a greater case hardeningeffect. This phenomenon at first seems difficult to understand since, inboth instances, a high oxygen potential exists at the air/oxideinterface for the oxidation treatment or at the oxide/Ti interface forthe treatment in accordance with the present invention. It is known thatoxidation of titanium is controlled by oxygen diffusion in the diffusionzone rather than in the oxide, since the diffusion coefficient foroxygen in TiO₂ is about 50 times that in α-Ti at the same temperature.Therefore, there is no reason to relate to the difference in thehardening effect between the process of the present invention conductedat 850° C. for a total time of 20.5 hours and a simple oxidationtreatment effected at 850° C. for 20.5 hours, to the diffusionresistance of oxygen passing through the oxide layer.

Without prejudice to the present invention, it is theorised that theabove phenomenon is caused by the retarding effect of nitrogen (from theair) on the diffusion of oxygen. During prolonged treatment in air, abuild-up of nitrogen atoms may occur at the oxide/metal interface (seeA. M. Chaze et al, Journal of Less-Common Metals, 124 (1986) pages 73 to84 ) and may act as a block on the inward diffusion of oxygen. In theabove described process according to the present invention, no furthernitrogen is admitted during vacuum treatment and the blocking effect istherefore much reduced.

The examples quoted above for the alloy Ti-6A1-4V, have been casehardened using process parameters that have substantially been optimisedfor that alloy. In order to demonstrate that the process is equallyapplicable to other alloys of titanium, a limited number of samples ofC.P titanium, Timet551 and Timet10-2-3 have also been treated. Thefollowing examples are for demonstration only and do not necessarilyrepresent an optimised process.

Samples of C.P titanium in the form of rectangular blocks of 20×10×10mm, cut from a 10 mm thick sheet, were used. The samples were degreasedand then thermally oxidised in air at 850° C. for 20-30 minutes. Aftercooling, the samples were subjected to a further heat treatmentoperation at 850° C. for 22 hours in a vacuum furnace (about 1×10⁻⁶Torr=about 1.3×10⁻⁴ Pa).

Samples of Timet551 in the form of rectangular blocks of 30×10×10 mm,cut from a 90 mm diameter bar, were used. The samples were degreased andthen thermally oxidised in air at 900° C. for 19 minutes. After cooling,the samples were subjected to a further heat treatment operation at 900°C. for 20 hours in a vacuum furnace (about 1×10⁻⁶ Torr=about 1.3×10⁻⁴Pa).

Samples of Timet10-2-3 in the form of rectangular blocks of 30×10×10 mm,cut from a 260 mm diameter forged disc, were used. The samples weredegreased and then thermally oxidised in air at 900° C. for 25 minutes.After cooling, the samples were subjected to a further heat treatmentoperation at 900° C. for 20 hours in a vacuum furnace (about 1×10⁻⁶Torr=about 1.3×10⁻⁴ Pa).

It was noted that, after thermal oxidation, the C.P and Timet551 samplesexhibited a grey appearance, whereas the Timet10-2-3 material exhibiteda black appearance.

As can be seen from FIGS. 4 and 5, the C.P and Timet551 hardnessprofiles exhibit the same type of sigmoid shape as FIG. 2 (OD) but 20 μmdeeper penetration in the case of Timet551 (c.f. FIG. 2); the slightlylower hardness and deeper penetration being attributed to the 20 hour900° C. diffusion step.

As can be seen from FIG. 6, the metastable β material has developed amuch deeper hardening compared with the α+β titanium alloys. The deeperpenetration of the oxygen can firstly be attributed to the higherdiffusivity of oxygen in the β phase (see Z. Liu and Welsch,Metallurgical Trans. A, Vol. 19A, Apr. 1988, pg1121-1125) and also to amuch thicker oxide layer which developed during step (a), compared withthe α+β titanium alloys.

In some alloys, the thermochemical treatment carried out in step (a)and/or step (b) of the case hardening process may alter themicrostructure and mechanical properties of the core material. In suchcases, a further heat treatment may be carried out after the casehardening process in order to restore or optimise the core properties.

It is important in the present invention that the scale formed duringstep (a) should remain adherent to the surface in order to provide theoxygen reservoir required for step (b). Depending on the alloy, theadhesion of the scale during step (a) can be affected not only by thetime and temperature employed but also by the nature of the oxidisingatmosphere and by the surface finish and geometrical shape of thesurface treated. When titanium is oxidised at around 850° C., the scaleformed is significantly more adherent if the oxidising atmosphere is airrather than pure oxygen, and a model has been proposed to explain thisas an effect of the presence of nitrogen. Our experiments have confirmedthe superiority of an air atmosphere over oxygen in this respect, and itis therefore not only more economical but also a technically preferredoption to use air as the oxidising atmosphere in step (a). The surfacefinish applied to all samples here described was obtained by finishingon 1200 grade SiC paper and this generally gave good adhesion.

It is to be understood that the case hardening process here describedresults in a relatively deep case of hardened material which enables itto withstand the sub-surface Hertzian stresses developed by high contactloads. The resultant surface has therefore a high load-bearing capacity,but this does not, of itself, confer good wear resistance to thesurface. In order to obtain a surface with low friction, which isresistant to scuffing and galling, it will be necessary to apply afurther layer or coating to the case hardened surface, or other surfacetreatment. Coatings, which have successfully been applied to the casehardened surface, include plasma nitriding, a diamond-like carboncoating, and the coating produced by the process described in ourcopending PCT Publication WO98/02595.

What is claimed is:
 1. A method of case hardening an article formed ofat least one material selected from the group consisting of titanium,zirconium, alloys of titanium, alloys of zirconium and alloys oftitanium and zirconium, said method comprising the steps of (a)heat-treating said article in an oxidising atmosphere at a temperaturein the range of 700 to 1000° C. so as to form an oxide layer on thearticle; and (b) further heat-treating the article in a vacuum or in aneutral or an inert atmosphere at a temperature in the range of 700 to1000° C. so as to cause oxygen from the oxide layer to diffuse into thearticle whereby to produce a sigmoid-shaped hardness profile.
 2. Amethod as claimed in claim 1, wherein the oxidising atmosphere containsboth oxygen and nitrogen.
 3. A method as claimed in claim 2, wherein theoxidising atmosphere in step (a) is air.
 4. A method as claimed in claim1, wherein the time for heat-treatment in step (a) is from 0.1 to 1hour.
 5. A method as claimed in claim 1, wherein the time forheat-treatment in step; (a) is from 0.3 to 0.6 hour.
 6. A method asclaimed in claim 1, wherein the heat-treatment in step (a) is effectedat atmospheric pressure.
 7. A method as claimed in claim 1, whereinsteps (a) and (b) are repeated at least once.
 8. A method as claimed inclaim 1, wherein the temperature in step (a) is 700 to 900° C.
 9. Amethod as claimed in claim 8, wherein the temperature in step (a) is 800to 900° C.
 10. A method as claimed in claim 1, wherein the temperaturein step (b) is 700 to 900° C.
 11. A method as claimed in claim 10,wherein the temperature in step (b) is 800 to 900° C.
 12. A method asclaimed in claim 1, wherein the heat treatment in step (b) is effectedat a pressure of not more than 1.3×10⁻² Pa (1×10⁻⁴ Torr).
 13. A methodas claimed in claim 12, wherein the heat treatment in step (b) iseffected at a pressure of about 1.3×10⁻⁴ Pa (1×10⁻⁶ Torr).
 14. A methodas claimed in claim 1, wherein the heat treatment in step (b) iseffected for a time in the range of 10 to 30 hours.
 15. An articleformed of a metal or alloy selected from the group consisting oftitanium, zirconium, alloys of titanium and alloys of zirconium, saidarticle having a hardened metallic case, strengthened by diffusedoxygen; wherein the article has a sigmoid-shaped hardness profile acrosssaid hardened case.
 16. An article as claimed in claim 15, wherein thedepth of the hardened case is greater than 50 μm.
 17. An article asclaimed in claim 15, wherein the depth of the hardened case is in therange 200 to 500 μm.
 18. An article as claimed in claim 15, furthercomprising a layer of low-friction material provided on top of thehardened case.
 19. A method of case hardening an article formed of atleast one material selected from the group consisting of titanium,zirconium, alloys of titanium, alloys of zirconium and alloys oftitanium zirconium, said method comprising the steps of (a)heat-treating said article in an oxidising atmosphere containing bothoxygen and nitrogen at a temperature in the range of 700 to 1000° C. soas to form an oxide layer on the article; and (b) further heat-treatingthe article in a vacuum or in a neutral or an inert atmosphere at atemperature in the range of 700 to 1000° C. so as to cause oxygen fromthe oxide layer to diffuse into the article, wherein the time forheat-treatment in step (a) is from 0.1 to 1 hour.
 20. A method of casehardening an article formed of at least one material selected from thegroup consisting of titanium, zirconium, alloys of titanium, alloys ofzirconium and alloys of titanium zirconium, said method comprising thesteps of (a) heat-treating said article in an oxidising atmospherecontaining both oxygen and nitrogen at a temperature in the range of 700to 1000° C. so as to form an oxide layer on the article; and (b) furtherheat-treating the article in a vacuum or in a neutral or an inertatmosphere at a temperature in the range of 700 to 1000° C. so as tocause oxygen from the oxide layer to diffuse into the article, whereinthe heat treatment in step (b) is effected at a pressure of not morethan 1.3×10⁻² Pa (1×10⁻⁴ Torr).